Masamitsu Haemori

Impact of Cu Electrode on Switching Behavior in a Cu/HfO2/Pt Structure and Resultant Cu Ion Diffusion

In a newly proposed switching device using polycrystalline HfO2 thin film with ion diffusion path, we have found that a Cu electrode could contribute to improved switching performance. Current–voltage measurements at room temperature revealed clear resistive switching, not accompanied by a forming process, in our Cu/HfO2/Pt structure. The current step difference from one state to the other one was in the order of 103–104, giving a sufficient on/off ratio. Voltage sweep polarity suggested that filamentary Cu paths were formed due to Cu ion diffusion and annihilated at the HfO2/Pt interface at reversed bias. This filament path formation and annihilation was the origin of the switching device performance.

Fabrication of Highly Oriented Rubrene Thin Films by the Use of Atomically Finished Substrate and Pentacene Buffer Layer

We report the remarkable effects of physical and chemical treatments of substrate surface on the physical vapor deposition of rubrene thin films. Highly c-axis oriented rubrene thin films were fabricated by combinatorial molecular beam epitaxy on atomically flat α-Al2O3 (0001) substrates, the surface of which was partially modified with pentacene buffer film. Rubrene thin films grown at room temperature on a sapphire substrate without pentacene buffer layer did not exhibit any X-ray diffraction pattern, whereas films deposited on a pentacene buffer layer exhibited peaks of c-axis orientation. Atomic force microscope images of the crystalline films show the steps of 1.3 nm height which correspond to the half c-axis length of the rubrene crystal. Preliminary, p-type operation was observed in bottom-gate field effect transistors using this rubrene film deposited on a pentacene buffer.

Bright blue phosphors in ZnO–WO3 binary system discovered through combinatorial methodology

We screened bright blue phosphor materials for a ZnO–WO3 binary system. The film was deposited by combinatorial pulsed laser deposition at room temperature and then annealed. The film composition changes continuously in a full-range from ZnO to WO3 on a single substrate. We obtained two results from the distribution of blue emission-intensity observed by cathodoluminescence of the film: (1) not only did the film have a W composition of 50 mol % (stoichiometric ZnWO4 as the well-known blue phosphor material) but also the W composition region from 45 to 60 mol % had the most emissive phase, and (2) two emissive phases other than the ZnWO4, which have not been described in the phase diagram, were found in the W composition region from 10 to 20 mol % and from 65 to 75 mol %.

An in-situ fabrication and characterization system developed for high performance organic semiconductor devices

We have designed and set up a fabrication and characterization system for organic devices which enables us to assemble all components of devices and to characterize the device properties without breaking the vacuum. Using this system, top and bottom contact C60 field effect transistors (FETs) were fabricated and their performance was characterized. The top contact FET exhibited a mobility as high as 1.4 cm2/(Vs), which was higher than the bottom contact FET.

Continuous wave infrared laser deposition of organic thin films

We developed a continuous-wave infrared laser molecular beam epitaxy (CW-IR-LMBE) optimized for the fabrication of organic semiconductor films. The crystal quality of these organic thin films deposited by CW-IR-LMBE was substantially the same as those deposited by thermal evaporation. Due to the possibility of quick switching of evaporation sources, CW-IR-LMBE is especially advantageous for rapid screening of composition, thickness, and fabrication parameters in materials and device optimization based on combinatorial technology.

Single grain and single grain boundary resistance of pentacene thin film characterized using a nanoscale electrode array

In this paper, we report on a less than 100-nm-wide nanoscale electrode array, with which the field-effect mobilities of a single grain and a single grain boundary in organic thin films are characterized. The method of fabricating the nanoscale electrode array and the evaluation results of the pentacene thin film are described. The nanoscale electrode array was fabricated by EB lithography and a liftoff process. The pentacene thin film was deposited by molecular-beam epitaxy (MBE). The resistances of a single grain and a single grain boundary were estimated to be around 10 and 100 MΩ, respectively, and the field-effect mobility was estimated to be around 1 cm2/(V s), which is almost comparable to the highest value ever reported. These results confirm that a single crystal is essential for high performance organic thin-film transistors.

Surface Nitridation of c-Plane Sapphire Substrate by Near-Atmospheric Nitrogen Plasma

The nitridation of c-plane sapphire substrates by near-atmospheric nitrogen plasma was investigated. The nitridation was carried out by irradiating the substrates directly (direct plasma) and remotely (remote plasma) with a flow of 400 sccm of generated nitrogen species at room temperature. After nitridation, the substrates maintained a clear step-and-terrace structure. X-ray photoelectron spectroscopy revealed clear differences in nitridation between the direct and remote plasma treatments. The substrate irradiated by the remote plasma showed mostly surface nitrogen termination, whereas the substrate irradiated by the direct plasma included numerous Al–N and O–N bonds.

Combinatorial synthesis of Cu/(Ta(x)Nb(1-x))2O5 stack structure for nanoionics-type ReRAM device.

Resistive random access memory (ReRAM) has been proposed as a new application for oxide materials. We have proposed a Cu electrode/dielectric oxide/bottom electrode stack structure as a potential ReRAM material that is compatible with the LSI process. Control of the switching voltage and the initial conductive filament formation process is beneficial for actual applications. To control the switching property by controlling the valence state of metals, we investigated the Ta-Nb binary oxide ((TaxNb1-x)2O5) system as a dielectric oxide layer using a combinatorial method. A combinatorial pulsed laser deposition method was used to fabricate the (TaxNb1-x)2O5 system systematically on a Pt/Si substrate. X-ray photoelectron spectroscopy revealed defect formation relating to Ta and the compensation of oxygen vacancies caused by a change in the valence number of Nb. As the Ta content decreased, there were a decrease in the threshold voltage of the low resistive state and an enhancement of the leakage current, meaning that the switching properties can be controlled by controlling the (TaxNb1-x)2O5 system.

Molecular-Wetting Control by Ultrasmooth Pentacene Buffer for High-crystallinity Organic Field-Effect Transistors

Organic thin film devices are of interest for a variety of forthcoming ubiquitous electronics applications. In order to build ubiquitous high-performance devices, it is necessary to fabricate crystalline thin films of various organic materials onto “ubiquitous substrates” that are dictated by applications. However, many organic thin films crystallize only on a limited selection of substrates. Unfortunately, promising organic molecules, which have a large overlap of pi-orbitals between molecules, cannot migrate freely on a substrate because of stronger cohesion between molecules than interaction between the molecule and the substrate. Therefore, enhancement of the molecule-substrate interaction, i.e. ‘molecular wettability’ should promote crystallization. We found that an ultrasmooth monolayer of pentacene (C 22 H 14 ), which can be grown on many general dielectric substrates, changes the molecular wettability of a substrate for other poorly wettable organic materials. We also demonstrate that a field effect transistor (FET) using a crystalline C 60 thin film on a pentacene-buffered substrate can have a mobility of 4.9 cm 2 /Vs, which is 5-fold higher than that of C 60 FETs without the buffer. Molecular wetting-controlled substrates can thus offer a general solution to the fabrication of high-performance crystalline plastic and molecular electronics.

Fabrication of Liquid Crystal Polymer Films and Their Passivation Effect for Organic Devices

We have investigated the fabrication of liquid crystal polymer (LCP) thin films by thermal evaporation, and pulsed laser deposition (PLD). Due to the carbonization of LCP, no films were fabricated by thermal evaporation. The laser desorption time-of-flight mass spectra of a LCP target exhibited that the main ablation species were the fragments of the LCP in the case of low laser fluence, indicating the possibility of film fabrication by PLD. Thin films fabricated by PLD using a KrF excimer laser have a pale yellow color and a surface roughness of 1 nm, as measured by atomic force microscopy. The spectra obtained by Fourier transform infrared spectroscopy exhibit a peak corresponding to carbonyl bond at approximately 1730 cm-1, indicating a polyester structure similar to that of the original LCP. The breakdown field of the films was 1 MV/cm. We demonstrate that this film acts as a passivation films for organic field-effect transistors.